The Atmospheric Infrared Sounder (AIRS) is a grating array infrared hyperspectral sounder with 2378 channels from 3.75 to 15.4 microns with spectral resolution 1200 to 1400 depending on the channel. AIRS was designed as an aid to weather prediction and for atmospheric process studies. It produces profiles of atmospheric temperature and water vapor. Because of its spectral coverage and spectral resolution it is sensitive to a number of trace atmospheric constituents including CO2, CO, SO2, O3, and CH4. The AIRS sensitivity, stability, and long life have led to its use in climate process studies and climate model validation, both of which place far more stringent requirements on calibration than weather forecasting does. This paper describes results from several special calibration sequences, originally developed for prelaunch testing, that have been used to monitor the AIRS calibration accuracy and instrument health on-orbit, including the scan mirror, space view response, and channel health. It also describes reanalyses of pre-launch calibration data used to determine calibration parameters. Finally, it shows comparisons of AIRS radiometry with two other hyperspectral infrared sounders presently in space—IASI and CrIS.